An increasing demand exists in several industries for large single crystals of various substances having high melting temperatures. To be useful these crystals must often be essentially free of structural defects such as large angle grain boundaries, inclusions, etc. Such industries include the electronics industry which wants large single crystals for economical mass production of semiconductor devices. Large single crystals also are in demand for use in lasers which employ, for example, crystalline laser rods or crystalline plates to close the emission end of gas lasers.
A widely used technique to grow single crystals is the Czochralski method. This method involves the melting of crystal growth stock in a crucible located in a furnace. A seed crystal having a predetermined orientation is dipped into the melt, and the heat input provided by the furnace to the melt is reduced while the seed crystal is rotated and slowly withdrawn from the melt.
Another widely used technique for growing single crystals is the Bridgman-Stockbarger process. Instead of withdrawing a seed crystal from a melt, a seed crystal is positioned at the base of the crucible into which melt stock is loaded. The crucible is heated to form a melt that varies in temperature from the seed temperature to higher temperatures upward into the melt. The crucible is then lowered slowly from a high temperature melt zone to a lower temperature cooling zone to induce crystal growth from the seed crystal at the base of the crucible to the top of the melt.
Still another technique generally referred to as vertical gradient freeze process maintains the crucible stationary while otherwise inducing vertical crystal growth in the crucible. For example, overall heating power may be lowered so as to move the melting point isotherm upward through the crucible. Another procedure involves use of a heat exchanger to extract heat from the base of the crucible. In general, it is desirable in the vertical gradient freeze process and also in the Bridgman-Stockbarger process to partially melt back the seed crystal to mitigate against a polycrystalline growth start.
Most if not all apparatus and methods currently being used commercially to grow single crystals produce crystal ingots having a cylindrical shape. If crystals in plate or slab form, or some other non-cylindrical form, are required for a particular application, these shapes would be cut from the raw cylindrical crystals. An attempt has been made to grow a single crystal of sapphire in plate/slab form using a rectangular crucible. This attempt involved a Czochralski type process wherein the seed is dipped into and then slowly pulled from the melt heated to a molten state by an RF heater. The Czochralski method, however, is believed to be incompatible with the growth of large crystals having a large dimension normal to the growth direction because of gravity induced defects resulting from the inability to provide bottom support for the weight of the crystal during growth.
Respecting known methods and furnaces presently being used commercially to grow single crystals of cylindrical shape, attempts to grow larger crystals give rise to problems of one sort or another such as that noted above with respect to the Czochralski method. One or more of these problems in significant part arise from the difficulty in maintaining an adequate (sufficiently high and axially directed) temperature gradient over the full cross-section of the increased diameter growth crucible. As the diameter of the crystal is increased, there comes a point where control of the growth process at the liquid-solid interface is no longer adequate. For single component crystals, in general, acceptable yields have been obtained for crystal diameters up to about 4 inches in diameter. However, as diameter is increased, the yields of single crystals progressively decrease; e.g., one might obtain just one 12 inch diameter crystal of acceptable quality out of a hundred growths.
In general, these known methods and furnaces for growing single crystals are generally not scalable. A scaled-up version of the furnace is unlikely to operate with performance equivalent to that of the smaller version. One reason for this is that as the diameter of the crystal is increased, the spacing between the center of the crystal and the controlled source of heat also is increased. As a result it becomes more difficult to control the temperature gradient at the center of the crystal-melt interface. Consequently, there is reduced control over the crystal growth process and increased likelihood of polycrystalline growth.